Field of the Invention
The present invention relates to pneumatic tires.
Description of the Related Art
In recent years, run flat tires including load support layers inside sidewalls have been developed and widespread. A highly hard crosslinked rubber is used for the support layers. Such run flat tires are called a side reinforcing type. In this type of a run flat tire, if the internal pressure is reduced due to puncture, the support layers can support the vehicle weight. Run flat tires allow for running for some distance even in a punctured state. Regarding run flat tires, various examinations have been made. An example of the examinations is disclosed in JP2010-137853 (US2010/0147439).
The tire disclosed in the above publication includes a carcass, first and second inserts, and an inner liner. The carcass includes a first reinforcing ply. The first and second inserts are located between the inner liner and the first reinforcing ply. In this tire, tension is easily applied to the first reinforcing ply. Since the reinforcing ply supports a load, a tensile load applied to the first and second inserts is minimized. Accordingly, a “soft” run on flat design is achieved in this tire.
When a tire becomes punctured so that the internal pressure thereof decreases, the tire itself supports the vehicle weight. Since a great load is applied to the tire, deformation of the tire is large. The deformation generates heat. Thus, when running in a state where a tire is punctured so that the internal pressure thereof decreases (hereinafter, referred to as run-flat running) is performed, the tire becomes very hot. The heat of the tire accelerates deterioration of the components of the tire, and thus there is a possibility that the components are damaged.
In a run flat tire, load support layers are provided to support the vehicle weight when the internal pressure of the tire decreases due to puncture. In view of the reason for adopting the support layers, it is ideal for the run-flat running if damage occurs from the support layer. However, in the run flat tire, damage may occur in a bead portion, not in a support layer portion. In this case, the damage tends to occur at a relatively early stage, and there is a possibility that the vehicle cannot run sufficiently in a punctured state.
Each bead of a tire includes a core and an apex. The core has a ring shape. The apex extends from the core outward in a radial direction. The apex is tapered outward in the radial direction. The apex is formed from a highly hard crosslinked rubber.
If deformation of a tire is suppressed, heat generation is also suppressed. A low degree of heat generation contributes to improvement in durability. In this respect, adopting a large apex may be considered in order to suppress deformation of a bead portion to ensure desired durability of this portion.
In manufacture of a tire, a combination of a core and an apex may be prepared as an intermediate component. However, the width of the apex is limited by the width of the core, so that a large apex cannot be adopted in some cases.
An apex may be prepared as an intermediate component. Normally, the apex is processed such that a cross-section thereof is triangular. The apex that has not been vulcanized is flexible. If the apex is kept in a state where the apex is wound on a drum or the like, the apex may become deformed. The deformed apex may influence the performance of a tire.
An apex may be formed by using a band-like strip. In this case, the strip is wound many times and the shape of an apex is adjusted, so that it is not easy to make only a specific portion thick.
On an apex composed of a single member, there are many structural limitations, so that it is difficult to ensure desired stiffness of a bead portion to suppress deformation of this portion.
An object of the present invention is to provide a pneumatic tire that achieves improvement in durability in a state where the internal pressure of the pneumatic tire is decreased due to puncture.